I would probably call them ‘boards’. Communities is too long of a word imo and it makes it sound like the people subscribed all have some kind of shared culture or relationship. That definitely happens in some cases (there are subreddits which have built a community around them) but it’s not universal.

What is wrong with using communities? By the way, in kbin they are called magazines.

I’m on kbin and they are called magazines, but I think communities are a better catch all. Everyone knows what you mean by that